The concentration of Al in a molten zinc bath (the weight % of Al to the entire molten zinc bath) in a molten zinc pot disposed in a continuous hot dip galvanizing line for a steel sheet affects the quality of a galvanized steel sheet, particularly, the quality of an alloy layer of base iron and zinc. Therefore, in order to stabilize the quality of the galvanized steel sheet, it is important to maintain the concentration of Al in the molten zinc bath at a constant level.
Hitherto, for the purpose of compensating the amount of molten zinc taken out of the molten zinc pot by a steel sheet, a zinc ingot containing Al is injected to the molten zinc pot from the above the molten zinc pot to maintain the amount of molten zinc in the molten zinc bath at a constant level and to roughly adjust the concentration of Al in the molten zinc (Patent Document 1).
In addition, a method is employed in which the concentration of Al in the molten zinc bath is measured by ICP analysis performed by drawing up a portion of the molten zinc in the molten zinc pot or an Al concentration meter installed in the molten zinc pot. Then, when the concentration of Al in the molten zinc bath is reduced, a Zn—Al alloy piece (so-called aluminum cake) having a higher concentration of contained Al than that of a zinc ingot containing Al is injected, controlled by an operator, into the surface layer of the molten zinc bath from the above the molten zinc pot, thereby finely adjusting the concentration of Al in the molten zinc. In general, the weight of the zinc ingot is tens to hundreds of kilograms, and the weight of the Zn—Al alloy piece (aluminum cake) for fine adjustment is about 5 to 10 kg.
Al in the zinc ingot containing Al and the Zn—Al alloy piece has a smaller specific gravity than zinc. Therefore, in a case where the zinc ingot containing Al or the Zn—Al alloy piece is injected in the above-described method, the concentration of Al at the bath surface of the molten zinc bath is increased, and thus, the surrounding of the bath surface is in a state of having a high Al concentration. On the other hand, the bottom portion of the molten zinc pot is in a state of having a low Al concentration, and thus bottom dross is likely to be generated and deposited on the bottom portion. The bottom dross rises due to stirring flow in the pot and adheres to the steel sheet when the sheet-threading speed of the continuous hot dip galvanizing line is in high speed. The bottom dross that adheres to the steel sheet is a cause for pressing flaws and degrades the product value of the galvanized steel sheet. Therefore, in the present, in order to avoid this problem, the upper limit of the sheet-threading speed is restricted, and the bottom dross is pumped out by regularly stopping facilities. The restriction on the sheet-threading speed and the regular stop of the facilities are the causes for degradation in productivity.
In addition, during the injection by the control of the operator as described above, the injection pitch is roughened, and an increase in the difference between a target Al concentration and an actually acquired Al concentration cannot be avoided. Accordingly, the quality of the alloy layer of the galvanized steel sheet is not stabilized, and insufficient alloying called half-baking or excessive alloying occurs, which is the cause for the degradation in product quality.